The connecting flange can thereby have a hollow closed profile, shaped in any way, whose individual flange walls extend straight or angled in a sectional view. Most of the time, the hollow profile of the connecting flange is constructed with a connecting flange wall going out radially from a duct flange wall lying in the plane of a duct wall, and with a supporting flange wall adjacent thereto and extending obliquely to the duct wall in various shapes.
With known air duct sections of this type, the supporting flange walls of the connecting flanges, form fitted in one piece, are fastened on the outside of the duct walls by spot welding. This additional procedure not only increases the cost of manufacturing but, in addition, the spot welded points are also susceptible to corrosion and must, preferably, be coated with an anticorrosive agent. Moreover, the attained fixing of the supporting flange wall is only at these points and, thus, not uniformly secure over the entire length of the profile.
Furthermore, air duct sections with flange profiles are known which have only one flange projecting at right angles from the duct wall, the outer edge of said flange is bent back at right angles parallel to the duct wall. The opposite bent back edges are pressed together by a profile clamp which is to be slipped on. Of course, with this specific embodiment, one does not have to spot weld, however, the slipping on of the profile clamp onto the bent back edges is extremely troublesome, in particular, then when the bent back edges are easily damaged during transportation and at the construction site, which occurs very frequently. Moreover, the stability of air ducts having connection flanges of this type is unsatisfactory, since the canted flanges at the ends of the air duct sections do not offer an adequate stiffening. Problems also arise during assembly if there is insufficient room for slipping on the profile clamps.